1. Field of the Invention
The present invention relates to a printing apparatus and a printing method that cause a print head having ink-ejecting nozzles to print on a print medium as it scans over the print medium.
2. Description of the Related Art
Inkjet printing apparatus of serial type uses a print head having a plurality of ink-ejecting printing elements (or nozzles) and performs a printing operation by repetitively alternating a print scan and a print medium convey operation, the print scan causing the print head to eject ink as it moves over the print medium. Among a plurality of nozzles there are some unavoidable variations in an ink ejection volume and in an ejection direction. These variations may result in a density unevenness and stripes showing up in a printed image. To alleviate these density unevenness and stripes, a multipass printing method such as disclosed in Japanese Patent Laid-Open No. H05-31922 (1993) has been known.
FIG. 31 is a schematic diagram showing a general multipass printing method disclosed in Japanese Patent Laid-Open No. H05-31922 (1993). Denoted P0001 is a print head which, for a simplicity of explanation, has 16 nozzles. The nozzles are divided into four nozzle groups—first to fourth group—as shown, each having four nozzles. P0002 represents a mask pattern which has its areas (or subpixels) preset either as printable (permitted to be printed) (shown in black) or as non-printable (not permitted to be printed) (shown in white). The mask patterns used by the individual nozzle groups are complementary to each other and, when overlapped together, complete the printing in an area corresponding to a 4×4-subpixel section.
Individual patterns at P0003 to P0006 show how an image is progressively completed as the print scan is repeated. Each time one print scan is finished, a print medium is conveyed a distance equal to the width of one nozzle group in a direction shown in the figure. Therefore, in the same area of the print medium (corresponding to the width of each nozzle group) an image is completed in four print scans.
With such a multipass printing performed, each area on the print medium is printed with a plurality of nozzle groups in a plurality of scans, so that variations characteristic of the nozzles and variations of print medium convey precision can be dispersed, reducing density unevenness and stripes.
Although the mask pattern P0002 has been shown in FIG. 31 as an example, a variety of effects can be produced by making some arrangements on the print-permitted subpixels in the mask pattern.
For example, Japanese Patent Laid-Open No. 2006-44258 discloses a technique that produces a uniform image with reduced graininess by determining a print-permitted subpixel arrangement based on repulsive force potentials to enhance the dispersion of printed dots.
In recent years, however, density changes or grayscale level changes caused by a print position misalignment (misregistration) among different print scans during the above multipass printing have come to be viewed as a new issue.
For example, when a 4-pass printing such as shown in FIG. 31 is performed, if an unexpected shift occurs during a print medium convey operation, dot groups printed in different print scans become misaligned with each other in print position. More specifically, if an undesired shift occurs during the convey operation between first pass and second pass over a unit area, the dot group printed in the first pass is placed out of alignment with dot groups printed in the second to fourth pass.
Such a print position misalignment, when it occurs, causes many of the dots, that are determined by the mask pattern to be printed at different positions in different print scans, to overlap each other. As a result, a complementary relationship among the dots collapses, lowering a dot coverage over the print medium (or area factor), which in turn reduces a grayscale level in the unit area. Further, if unit areas with such print position misalignments and those with no such misalignments are intermingled on the same print medium, the above phenomenon is perceived as a density unevenness.
Mask patterns disclosed in Japanese Patent Laid-Open No. 2006-44258 incorporate special arrangements to place dots at equal intervals as much as possible and with as high a dispersion level as possible because a priority is given to a graininess observed in normal state without any print position misalignment. So, if a print position misalignment occurs among different dot groups, as caused by unexpected shifts during convey operations, the dot dispersion is greatly lost, making image impairments brought about by density reductions easily noticeable.
Other technique to deal with the print position misalignment is disclosed, for example, in Japanese Patent Laid-Open No. 2001-322262. This technique provides a mask pattern with subpixels where dots are permitted to be printed overlappingly, in order to prevent density reductions and bandings even if an undesired print medium shift occurs during its convey operation. With this method, if a print position misalignment occurs, overlapping dots are separated from each other to raise an area factor to some extent. This method therefore can be expected to prevent possible density reductions.
However, in the method of Japanese Patent Laid-Open No. 2001-322262, it is simple matter that subpixels in mask patterns where dots are permitted to be printed are arranged to overlap each other. So if print data does not exist in those subpixels, overlapping dots are not printed. That is, in an image that has a large proportion of print data in other than the overlapping subpixels, the method cannot produce its intended effect.
As described above, not one conventional technique can avoid the problem of density variations in the event of a print position misalignment caused by unexpected shifts during print medium convey operations or the like.